Method of making lithographic master

ABSTRACT

A lithographic plate comprises a hydrophilic backing having an oleophilic solvent-removable coating which becomes solventresistant when heated in reactive association with an oxidizing agent. The coating comprises a polyphenolic reducing agent together with a lesser amount of chemically inert film-forming binder.

United States Patent [191 Mukherjee [111 3,811,886 [451 May 21, 1974 [22] Filed: May 10, 1972 [21] Appl. No.: 252,097

Related U.S. Application Data [62] Division of Ser. No. 847,393, Aug. 4, 1969, Pat. No.

[52] U.S. Cl 96/33, 101/467, 250/317 [51] 1nt.'Cl. G031 7/02, B41m 5/00, B4lm 3/00 [58] Field of Search 101/467; 250/316, 317,

[56] References Cited UNITED STATES PATENTS 3,080,254 3/1963 Grant 250/317 3,168,864 2/1965 Brand] et a1. 101/467 3,299,807

l/l967 Evensen 101/467 Primary Examiner-David Klein Attorney, Agent, or Firm-Alexander, Sell, Steldt & DeLal-lunt [57] ABSTRACT A lithographic plate comprises a hydrophilic backing having an oleophilic solvent-removable coating which becomes solvent-resistant when heated in reactive association with an oxidizing agent. The coating comprises a polyphenolic reducing agent together with a lesser amount of chemically inert film-forming binder.

2 Claims, No Drawings lesser amount of chemically inert film-forming binder.

Oxidation of the reducing agent at image-defining areas results in a significant increase in solvent resistance at those areas, permitting the remaining unreacted portions of the coating to be selectivelyremoved to expose the underlying hydrophilic'surface.

Oxidation at image areas is accomplished by reaction with metal soap oxidizing agents, of which silver behenate is a presently preferred example. The reactant may be applied only at image areas, e.g., in the form of an ink, but preferably is supplied coextensively with the reducing agent in the-same or a'separate layer, with subsequent localized image-forming reaction being accomplished by localized heating, catalysis, or other treatment. The silver salt in particular provides a visibly distinct black image so that the subsequent solvent removal of the non-imaged portions of the coating is facilitated. Y

Lithographic plates of the invention which contain both'silver salt and reducing agent may be imaged in a number of ways. One procedure involves the application of a thermographic heat pattern to the plate coat ing. In another procedure a light-sensitive intermediate is first exposed to a light-pattern and then heated against the ,plate.- In a further and presently preferred method the plate is itself light-sensitive and is exposed to a light-pattern and then heated to develop a solventresistant image. Still other procedures may be used. In all cases the still soluble unreacted portion of the coating is washed away to expose the hydrophilic substrate surface, while the reacted portion is solvent-resistant and remains affixed to the'substrate to provide oleophilic ink-receptive printing areas.

' The heat-reactive oleophilic coatings containing the silver salt and the reducing agent are best applied as liquid. compositions in a volatile liquid vehicle which is a solvent for both the reducing agent and the binder. The latter component is present only in minimum amount required to prevent flaking or powdering of the coating, and in minor proportion with respect to the former. Conversion of the reducing agent to a solvent-resistant form then protects the soluble binder from theeffects of the solvent.

Polyvinyl butyral is apreferred binder for these compositions and particularly for the light-sensitive varieties, since it serves to improve the sensitometric properties of the coatings as well as to provide effective anchorage to the hydrophilic substrate. It is readily soluble in alcohols and alcohol mixtures in which the polyphenol reducing agents are likewise soluble, so that alcoholic solvent mixtures are preferred with these compositions. Other binders which-have been found useful include polyvinyl alcohol, chlorinated polyethylene, and copolymer of hydroxypropylmethacrylate and normal butylmethacrylate. In all cases the amount of soluble non-reactive binder is kept as low as possible and in no case amounts to more than about three-fourths the amount by weight of the reducing agent.

Since the film-forming material is initially applied from solution in an organic solvent or solvent mixture, the lithographic master is most conveniently prepared by removal of the background portions of the coating with the same solvent. The solvent power of the original solvent may in some cases be more than adequate for the materials dissolved, and it is therefore frequently found desirable to use less active solvents or solvent mixtures in the development process. Thus, water-miscible alcohol or ketone solvents may be diluted with water to lower the solvent power of the mixture to a point just sufficient for removal of the background portions of the coating, thereby increasing the ability of the reacted image portions to resist the solvent mixture.

The reducing agent is found to undergo substantial increase in molecular weight and to become strongly solvent-resistant when oxidized. Compounds which contain at least two monohydroxyphenyl groups and 2,2'-methylenebis (fitertiarybutylphenol) F 5 wmmQwQwwmx I a Ha 2,2'-methylenebis (LmethyI-fi-tertiarybutylphenol) 1,1-bis-(2-11ydroxy-3,6-dimethylphenyl)-3,5,5-trlmethy1hexane 2,6-methylenebis-(2-hydroxy4i-tertiarybutylbmethylphenyl)-4- (can, (0H

4,4-thlobis (fl-tertiary butyl-o-cresol) OCH polyvinyl (Z-hydroxy-3-methoxybenzal) repeating structure 12:00 2:6 0H; CH: 21 Ha D-cresol-aoetaldehyde novolak resin wherein n averages about 4 wherein n is l or 2 These poly(monohydroxyphenyl) compounds are characterized by a methyl or hydrogen radical ortho or para to the hydroxyl radical, by solubility in volatile organic solvents, and by the ability to undergo oxidative coupling.

Hydroquinone is an example of a well-known reducing agent for silver which is not useful in the present invention. It contains two hydroxyl radicals on a single benzene nucleus, and on oxidation forms a stable quinoid structure.

Compositions and coatings containing poly(monohydroxyphenyl) compounds as above identified have pre-- viously been described; but in no case of which I am aware was the coated sheet material useful for the purposes here set forth. Thus Reitter US. Pat. No. 3,218,166 suggests various phenols, including one example of a polyphenol, namely 4,4'-methylenebis-(2,6- ditertiarybutylphenol), in combination with silver behenate in a coating on paper. The phenolic reducing agent is present in less than the amount needed to produce a dense image. The film-forming binder is present in an amount at least equal to the amount of the reducing agent and preferably in much greater amount. The sheet produces excellent copies of graphic originals when employed as directed in the patent, but does not provide the required differential solubility nor produce offset masters when processed according to the principles of the present invention. Again, Owen US. Pat.

No. 2,910,377 describes a heat-sensitive copysheet which may contain silver behenate and a reducing agent for silver. One of the reducing agents named, namely 4,4-dihydroxybiphenyl, is a polyphenol; but this compound forms a soluble stable quinoid structure 5 rather than a solvent-resistant high molecular weight product when oxidized. The copy-sheets of this patent likewise fail to provide an offset master when processed as herein taught.

Since solvent resistance is achieved by oxidation of the reducing agent, it is highly desirable that the oxidation reaction be as complete as possible. For this reason it is preferred to use stoichiometric proportions of the silver salt or other oxidizing agent and the poly(- monohydroxyphenyl) reducing agent, although some variation from these proportions may be tolerated, particularly for compositions containing those reducing agents which attain highest molecular weight or otherwise achieve greatest solvent resistance. ln some cases the ability of the reducing agent to form a solventresistant product is sufficient to permit inclusion of small amounts of other reducing agents which are much less effective in this respect or which are oxidized to fully soluble end products.

The following specific Examples will serve further to illustrate but not to limit the invention.

EXAMPLE 1 A slurry of silver behenate in isopropyl alcohol is homogenized by being forced through an orifice. To grams of the resulting suspension, containing 14.5% by weight of the silver soap, is slowly added .5 ml. of a 4% solution of mercuric bromide in isopropyl alcohol, with constant stirring and under darkroom conditions. One- 5 half gram of polyvinyl butyral is added and the mixture is ballmilled for three hours. Separately 1.5 gm. of pcresol-formaldehyde novolak resin and 0.25 gm. of l(2H)-phthalazinone is dissolved in 20 ml. of isopropyl alcohol and this solution, together with 2 ml. of 0.05% solution of sensitizing dye in acetone, is mixed with the soap suspension. Finally 8 gm. of a 5% solution of polyvinyl alcohol in water is added together with 20 ml. of additional water; again with thorough mixing and protected from light.

The mixture is coated on a hydrophilic silicatetreated aluminum panel at a coating orifice of three mils, and dried. The sheet is exposed to a light-image at 100 foot candle seconds and a visible image is developed in the light-struck areas by heating at 145 C. for a few seconds. The plate is then carefully scrubbed with a 1:1 mixture of isopropyl alcohol and water. The unexposed and undarkened portions of the coating are completely removed to expose the hydrophilic substrate. The black ink-receptive image areas remain on the plate, which is then placed on the press, inked, and used in printing a number of copies by the lithographic offset process.

Light-sensitive heat-developable coated sheet materials of similar composition but employing different reducing agents and which do not possess the separability characteristics of the coatings here described are the subject of Morgan et al. US. application Ser. .No. 693,714 filed Dec. 27, 1967, now US. Pat. No. 3,457,075.

The novolak resin is a thermoplastic alcohol-soluble acid-catalysed resinous condensation product of one mol of p-cresol and 0.8 mol of formaldehyde. The sensitizing dye is 3-allyl-5-[3-ethyl-(2- napthoxazoylidene )-ethylidene]- l -phenyl-2- thiohydantoin. The silver behenate, prepared by precipitation with silver nitrate from an aqueous solution of the sodium soap of commercial behenic acid, analyzes 25.2% silver.

Substitution for the novolak resin of 2 gm. of the compound, 1 ,1-bis-(2-hydroxy-3,S-dimethylphenyl)- 3,5,5-trimethylhexane gives a sensitive coating which produces a visible and solvent-resistant image when exposed to 1,000 foot-candles and heated at 145 C. and provides increased press life as compared with the novolak resin coating.

EXAMPLE 2 posed to a light-image for.50 foot candle seconds followed by heating for seconds at 122 C. The nonlight-exposed areas are preferentially darkened and rendered solvent-resistant; very little darkening is observed at the light-struck-image areas. Washing with a mixture of equal parts isopropyl alcohol and water removes the coating at the light-exposed areas, leaving the darkened and inkreceptive background areas. The resulting plate is placed on the press, inked, and used in making copies by offset printing.

EXAMPLE 3 A mixture of 33 grams of the silver behenate homogenizate described under Example 1 and 5 ml. of a 4 percent solution of mercuric bromide in isopropyl alcohol is ball milled until-uniform. To the mixture is added a. solution of 0.25 gm. phthalazinone and 1.5 gm. of p-cresol-formaldehyde novolak resin in ml. of ethyl acetate, together with 2 ml. of a 0.05% solution of sensitizing dye in acetone. The resulting mixture is coated on silicated aluminum at a coating thickness of 3 mils and dried, all under darkroom conditions.

Separately a thin heat-resistant polyester film is coated with a solution of 0.2 part by weight of 4-methoxyl naphthol, 0.088 part of erythrosin and 10 parts of ethyl cellulose in 90 parts of methylethyl ketone, applied at a 3 mil thickness anddried, to form alight-desensitizable' intermediate as described in Workman US. Pat. No. 3,094,417.

The intermediate is placed against'a printed original and exposed to light to the extent just sufficient to desensitize the coating at the non-printed areas. It is then removed from the original and placed against the coated surface of the plate, and the composite is heated for 10 seconds at 130 C. Removal of the intermediate discloses ablack image on the plate corresponding to the printed image of the original. The plate is carefully washed with a mixture of equal parts of isopropyl alcohol and water. The unprinted background portions of the coating are thereby removed, exposing the hydrophilic substrate and leaving the solvent-resistant inkreceptive image areas Copies are produced from the plate by the lithographic offset printing process.

EXAMPLE 4 A blend of 25 gm. of a homogenizate of 20 parts silver behenate in parts 2-propanol, 0.5 gm. of polyvinylbutyral, and 0.25 gm. of phthalazinone is prepared by grinding in a ball mill. A solution of 1.5 gm. of pcresol-formaldehyde novolak resin and 0.5 gm. of phenol-blocked polyol-isocyanate resin (Mondur S) in 10 ml. of 2-propanol is added. The smooth mixture is coated through a 3 mil coating orifice onto a polyester film which has previously been given a hydroph'ilic surface coating of colloidal silica in Vinylite VAGH partially hydrolyzed copolymer of vinyl chloride and vinyl acetate. The dried coating is light-resistant and heat-sensitive. A graphic original printed with black infra-red-absorptive characters on a thin white paper is placed against the coated plate with the unprinted surface in contact with the coating and is then briefly exposed to intense radiation rich in infra-red. Removal of the original reveals a black copy of the original image. Che plate is washed with a mixture of isopropyl alcohol and water to remove the coating and expose the hydropi'ime'iadarsin-raee at background areas: leaving the solvent-resistant and ink-receptive image. Copies are produced by the lithographic offset process.

Equally effective lithoplate masters are prepared in the same manner on a paper backing having a hydrophilic surface coating of clay and insolubilized casein or carboxymethylcellulose binder.

EXAMPLE 5 An aluminum plate having a hydrophilic silicatetreated surface is coated with a solution of 1 gram of polyvinyl butyral resin, two grams of 4,4-thiobis-(6- tertiarybutylorthocresol), and 0.25 gram of phthalazinone in 15 ml. of methanol, applied at a thickness of 2 mils, and dried.

Various markings are applied to the coated surface using inks containing respectively silver behenate, silver phthalate, mercuric stearate, ceric behenate, and ferric stearate as suspensions in acetone or methanol. Preferably, polyvinyl butyral is incorporated in such inks as a binder and viscosity increasing agent. The plate is heated and the marked areas change to a black color. Washing with a mixture of water and isopropyl alcohol removes the unmarked portions to expose the hydrophilic sub-surface. The marked portions are oleophilic and the plate is useful as a lithographic master.

EXAMPLE '6 A mixture of 25 gm. of 20% silver behenate suspension in isopropyl alcohol, one gm. of polyvinyl butyral resin, two gm. of 4,4'-thiobis(6-tertiarybutyl-oscresol) and 20 ml. of isopropyl alcohol is applied to a silicated aluminum panel and dried. Markings are applied to the surface of the coating with a marking fluid consisting of a solution of phthalazinone (a catalyst for the oxidation reaction) in a volatile solvent. Heating of the marked plate results in development of a black image and of sufficient solvent resistance to permit separation of image and background areas by washing with isopropyl alcohol-water mixtures. The plate is useful as a lithographic master.

The phthalazinone as used in Example 6 is a catalyst for the oxidation-reduction reaction. Another useful catalyst which can be used in lithographic plates of the invention comprises silver nuclei, as described in the above-mentioned Morgan et al. patent (U.S. Pat No. 3,457,075). The silver nuclei are formed when photosensitive silver halide which is already present or otherwise formed in situ in the oleophilic layer of the lithographic plate is exposed to light. Such silver nuclei are well known as catalysts for such oxidation-reduction reactions.

What is claimed is as follows:

1. A method of making a lithographic master from a lithographic plate comprising the following steps:

a. providing a lithographic plate comprising a hydrophilic substrate bearing a thin oleophilic ink? receptive layer, said oleophilic layer being readily removable by washing with a solvent mixture comprising water and an organic solvent selected from the group consisting of water-miscible alcohols and ketones, said layer containing as an essential ingredient an alcohol soluble, oxidizable, low molecular weight poly(monohydroxyphenyl) aromatic organic compound soluble in water-miscible alcohols and ketones and having a hydrogen or methyl radical in ortho or para positions to the hydroxyl radical and which on oxidation does not form a stable quinoid structure, together with a lesser amount by weight of a chemically inert film-forming binder, said coating, when briefly heated with a quantity of water-insoluble silver soap just sufficient to oxidize all of said compound and in the presence of phthalazinone as a catalyst for such oxidation reaction, being rendered resistant to attack by said solvent mixture;

b. heating the said oleophilic layer at image areas and in reactive association with an oxidizing agent in amount just sufficient to cause oxidation of all of said compound in said areas and continuing said heating for a time sufficient to produce significant resistance to said solvent mixture, and;

c. washing said plate with a mixture of water and an organic solvent selected from the group consisting of water-miscible alcohols'and ketones to remove the said oleophilic layer only at the unheated areas.

2. Method of making a lithographic master from a lithographic plate comprising the following steps:

a. providing a lithographic plate comprising a hydrophilic substrate bearing a thin oleophilic inkreceptive layer, said oleophilic layer being readily removable by washing with a solvent mixture comprising water and an organic solvent selected from the group consisting of water-miscible alcohols and ketones, said layer containing as an essential ingredient an alcohol-soluble, oxidizable, low molecular weight poly(monohydroxyphenyl) aromatic organic compound soluble in water-miscible alcohols and ketones and having a hydrogen or methyl radical in ortho or para positions to the hydroxyl radical and which on oxidation does not form a stable quinoid structure, together with a lesser amount by weight of a chemically inert film-forming binder, said coating further containing water-insoluble silver soap in an amount just sufficient to oxidize all of said organic compound, wherein about 1 to about 10 percent of said silver soap is converted to photosensitive silver halide,

b. exposing said plate to a light-image,

c. heating the exposed plate to develop a visibly distinct image of reduced solubility, and

d. washing said plate with said solvent mixture to remove said layer only at non-imaged areas. 

2. Method of mAking a lithographic master from a lithographic plate comprising the following steps: a. providing a lithographic plate comprising a hydrophilic substrate bearing a thin oleophilic ink-receptive layer, said oleophilic layer being readily removable by washing with a solvent mixture comprising water and an organic solvent selected from the group consisting of water-miscible alcohols and ketones, said layer containing as an essential ingredient an alcohol-soluble, oxidizable, low molecular weight poly(monohydroxyphenyl) aromatic organic compound soluble in water-miscible alcohols and ketones and having a hydrogen or methyl radical in ortho or para positions to the hydroxyl radical and which on oxidation does not form a stable quinoid structure, together with a lesser amount by weight of a chemically inert film-forming binder, said coating further containing water-insoluble silver soap in an amount just sufficient to oxidize all of said organic compound, wherein about 1 to about 10 percent of said silver soap is converted to photosensitive silver halide, b. exposing said plate to a light-image, c. heating the exposed plate to develop a visibly distinct image of reduced solubility, and d. washing said plate with said solvent mixture to remove said layer only at non-imaged areas. 